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Scientists say there is a distant group of objects, made of rock and ice, that forms a cloud-like region surrounding our Solar System. It is a cloud of comet-like objects orbiting far away from the Sun. Even though the comets are very widely scattered from each other, there many millions of them. The total mass of all these comets may be up to 100 times the mass of the Earth. The Oort Cloud is named after a Dutch astronomer Jan Oort who took the original idea, improved upon it and made it widely known.

This diagram shows about how far away the Oort cloud might be compared to the planets of the Solar System. Start in the upper left frame, then follow the pictures clockwise. Each picture shows a bigger volume of space.

As a comet makes several passes through the solar system, the Sun slowly melts and vaporizes the ice and only little bits of solid debris are left behind. But if the comets are all destroyed when they pass through the system, then new comets will need to appear. Otherwise we would not see any more comets. Jan Oort used the idea of the Oort cloud to explain why new comets keep appearing.

If you can imagine the distance from the Earth to the Sun, then the comets in the Oort cloud are 50,000 to 100,000 times further away! That is 1,000 times further away from the Sun than is Pluto, and about one fourth the distance to the nearest neighboring star—Proxima Centauri. Light takes a year to travel from the Sun to the outer edge of the Oort Cloud.

The Oort cloud objects may have started closer to the Sun during the Solar System's formation. When they passed near the gas giants, the gravity of those planets hurled the objects into very distant orbits. The Oort cloud objects were sent in all directions, making the Oort cloud ball-shaped instead of disk-shaped. The gravity of passing stars also made the orbits of these objects more circular, and pulled them further from the Sun. But sometimes the gravity of other far away stars can send the objects hurtling back toward the Sun. These become the comets.

An object named Sedna has been discovered that may belong to the Oort Cloud (although it is actually between the Kuiper Belt and the Oort Cloud.) It is from 1,180 to 1,800 km across. Its orbit stretches from 76 to 928 times further from the Sun than does the Earth's. Sedna orbits the Sun about once every 11,250 Earth years. The last time Sedna was where it is now in its orbit, Earth's last Ice Age was ending! Some scientists think that Sedna should be included in the Kuiper Belt, making the belt bigger.